1. Field of the Invention
This invention relates to a method and apparatus for anchoring a ribbed bar in a rock formation, and more particularly, to a method and apparatus for anchoring a ribbed bar in a bore hole by means of a chemical anchor, a mechanical anchor, or a combination chemical and mechanical anchor.
2. Description of the Prior Art
It is well known in the art of mine roof support to tension bolts anchored in bore holes drilled in a mine roof to reinforce the unsupported rock formation above the roof. Conventionally, a hole is drilled through the roof into the rock formation. The end of the bolt is positioned in the rock formation and anchored by either engagement of an expansion shell assembly on the end of the bolt with the rock formation or adhesively bonding the bolt by a resin mixture to the rock formation surrounding the bore hole. The resin mixture penetrates into the surrounding rock formation to adhesively unite the rock strata and to firmly hold the bolt in position in the bore hole. The resin mixture fills the annulus between the bore hole wall and the bolt along a substantial length of the bolt. With the expansion shell assembly it is also known to chamfer the end of the bolt so that the threads on the end of the bolt that receive the expansion shell assembly terminate at a point spaced from the chamfered end of the bolt. This arrangement facilitates dislodging of a stop means positioned within the expansion shell assembly.
U.S. Pat. No. 3,702,060 discloses an expansion shell assembly that includes a resin container which is fixed to the end of an expander positioned within an expansion shell. The container is ruptured after the shell begins to expand. Rotation of the bolt mixes the resin components, and the resin mixture surrounds the shell to embed the shell in the cured resin to bond the shell to the rock strata. When the resin is fully cured, a nut on the end of the bolt opposite a roof plate on the bolt is rotated to bring the roof plate to its fully seated position against the mine roof to fully tension the bolt.
U.S. Pat. No. 3,979,918 discloses a chemical anchor rock bolt and a method of installing an anchor bolt in a bore hole. The rock bolt is provided with a clamping member or nut which is threadedly engaged to the end of the bolt extending beneath the mine roof. The nut is crimped or pressed to remain in fixed engagement with the threaded end of the bolt during mixing of a chemical anchor resin positioned in the bore hole. Upon setting of the resin and on application of an increased driving torque to the nut, the nut is tightened on the rock bolt as required.
Combining bolt tensioning and resin bonding of a mine roof bolt in a bore hole is disclosed in U.S. Pat. No. 4,051,683. The device disclosed in this patent utilizes an internally threaded coupling which is connected at one end to a rebar anchored within the bore hole by the mixed and cured resin. A bolt connected to the other end of the coupling includes a bearing or roof plate advanced into abutting relation with the mine roof. A stop means provided in the coupling limits axial advancement of the bolt into the coupling to prevent relative rotation of the coupling and the bolt as the assembly is rotated to break the resin cartridge and mix the resin components. When the resin cures the rebar above the coupling is adhesively bonded to the rock formation. Thereafter rotation of the bolt in the coupling fractures the stop means to permit the bolt to move upwardly in the bore hole so that sufficient torque is applied to the bolt to tension the bolt.
Similar devices which utilize a rod anchored within the drill hole by resin bonding and connected to a bolt by a coupling are disclosed in U.S. Pat. Nos. 4,122,681 and 4,192,631. The coupling disclosed in U.S. Pat. No. 4,122,681 has an internally threaded bore in which a portion of the threads are deformed to permit initial rotation of both the rod and the bolt within the bore hole. These devices rely upon the bonding of the elongated rod to the rock formation by the resin mixture. They do not utilize a mechanical anchor.
U.S. Pat. Nos. 4,160,614 and 4,162,133 disclose a mechanical anchor in combination with resin bonding of a bolt in a rock formation. Rotation of the bolt with the mechanical anchor attached to the end thereof in a first direction effects mixing of the resin components of a ruptured cartridge. An anti-rotation device prevents relative rotation between the camming plug and the bolt so that the plug is not threaded off the end of the bolt during mixing of the resin components. With this arrangement the resin components are thoroughly mixed before the shell is expanded. After a period of time sufficient to allow mixing of the resin and before the resin hardens, the direction of rotation of the bolt is reversed to disengage the anti-rotation device. The camming plug is then free to advance downwardly on the bolt and expand the shell into gripping engagement with the wall of the bore hole.
U.S. Pat. No. 4,295,761 discloses an elongated anchor which is inserted in a bore hole after insertion of a breakable adhesive resin or grout containing cartridge. The anchor has a threaded end to be received in the threaded portion of a roof nut. The roof nut includes a frangible disc positioned in the threaded portion which is held in place by crimped lip portions. The frangible disc forms a stop wall to stop rotation of the nut relative to the anchor when the nut and anchor are assembled at a first relative axial position. When the anchor is restrained against rotation in the bore hole by setting of the adhesive resin and a torque above a selected threshold is applied to the nut, the crimping is dilated and the disc is expelled to accommodate further threaded insertion of the nut onto the anchor.
U.S. Pat. No. 4,347,020 discloses a mine roof bolt assembly including a shank having anchoring means at one end for securing the shank in the top of a bore hole. The opposite end of the shank is threaded. A bolt head having a central bore and a first threaded bore portion threadedly engages with the threaded end of the shank. A stop plug is threaded into a second threaded bore portion in the bolt head and abuts an end of the shank to initially limit penetration of the shank into the bore of the bolt head. The bolt head is rotated by rotating means having a predetermined torque limit, thereby moving the bolt head longitudinally with respect to the shank and exerting a force on the stop plug. The stop plug strips the threads from the second threaded bore portion, thereby permitting uninterrupted passage of the shank through the bore.
U.S. Pat. Nos. 4,413,930 and 4,419,805 disclose a mechanical anchor including an expansion shell and a camming plug positioned in the shell and threaded onto the end of a mine roof bolt. The mechanical anchor is inserted in a bore hole drilled in a rock formation with a bonding material advanced by upward movement of the bolt to the top of the bore hole. A stop means associated with the mechanical anchor prevents movement of the camming plug on the bolt as the bolt is rotated to effect mixing of the bonding material. The stop means is displaced from the mechanical anchor when the mixed bonding material exerts a force resisting rotation of the camming plug with the bolt, allowing the camming plug to move downwardly on the bolt to expand the expansion shell fingers in the bore hole to anchor the bolt.
U.S. Pat. Nos. 4,516,885 and 4,518,292 disclose a bolt assembly for securing a bolt in a bore hole having bonding material therein including an expansion shell and a camming plug positioned in the shell and threaded onto the end of a mine roof bolt. The expansion shell fingers are arranged to expand outwardly to engage the bore hole wall by longitudinal movement of the camming plug relative to the threaded end of the bolt and the expansion shell. Means is provided for mixing at least a portion of the bonding material upon rotation of the bolt in a preselected direction while maintaining the expansion shell fingers in an unexpanded condition. Means is also provided for moving the camming plug axially on the threaded end of the bolt and relative to the expansion shell fingers as the bolt is rotated in the same preselected direction to expand the expansion shell fingers and anchor the bolt in the bore hole.
U.S. Pat. No. 4,611,954 discloses an apparatus and method for anchoring a bolt in a bore hole including a tensioning plug and expandable sheath assembly threadedly engaged to the end of an elongated bolt. A ring is affixed to the inner wall of the plug adjacent the plug's threaded portion. The end of the bolt abuts the ring upon assembly, thereby causing the bolt and assembly to rotate together upon application of a torque to the bolt. This rotation effects mixing of bonding material placed in the bore hole ahead of the assembly. As the mixed bonding material begins to harden, rotation of the assembly is resisted, and the bolt deforms the ring by cutting threads therein. Continued rotation of the bolt causes the plug to travel axially along the bolt so as to expand the sheath extensions into contact with the bore hole wall.
U.S. Pat. No. 4,666,344 discloses a truss system and components thereof to support rock formations in underground mines, caverns, storage vaults and so forth. The truss brackets utilized in the truss system are designed so that a minimum or no force is exhibited when the tie rods connected between associated truss brackets are tightened down. The various truss brackets are maintained in abutting relation with the mine roof by rock bolts secured in place by a conventional resin anchor. The tie rods and rock bolts each have widely spaced continuous threads to receive a locking nut. The locking nuts are utilized to secure the various truss brackets to the rock bolts and to tension tie rods connected between associated truss brackets.
While it has been suggested by the prior art roof bolt systems to anchor a roof bolt in a bore hole by a combination of resin bonding and mechanical anchoring, the roof bolts presently utilized require a threaded end portion which must be machined onto the end of the standard roof bolt before installation. Threading the end of a roof bolt is a costly and time consuming process. Since the end of the roof bolt must be threaded before installation, roof bolts having a predetermined, fixed length must be threaded at a location distant from the mining operation, and thereafter shipped to the mine for use. Where a rebar is used as a part of the roof bolt system, preparing the ends of the rebar is an even more expensive and time consuming process. To provide a rebar with a threaded end portion, the rebar end portion must first be swaged and thereafter threads machined on the swaged end portion.
As an improvement over the presently available roof bolt systems, this invention discloses a method and apparatus for anchoring a ribbed bar in a bore hole by means of either a chemical anchor, a mechanical anchor, or a combination of chemical and mechanical anchoring. The ribbed bar utilized in this invention does not require threading before the ribbed bar can be used in a roof bolt system.
As is known in the art, rolled bars are commercially available which have a helically extending partial thread roll-formed on the bar outer surface. The extreme difficulty of roll-forming a complete thread on the outer surface of a bar utilizing presently available hot or cold rolled forming processes is well known. However, the technology is presently available for roll-forming a partial thread on the outer surface of a bar or rod. U.S. Pat. No. 3,561,185 discloses a ribbed bar having helically extending rib segments disposed on opposite sides of the circumference of the bar wherein the rib segments extend to their full height over approximately one-third of the circumference of the bar and the end surfaces of each rib segment merge with the outer surface of the bar. Since each bar is rolled to form a partial thread on the bar outer surface, bars positioned end to end may be connected by a coupling having internal threads which mate with the helically extending rib segments formed on the ribbed bars. The suggested use for this ribbed bar is as an anchor member for prestressing concrete.
Bars having partial threads roll-formed on their outer surface have also been used in mining applications as roof supports. For example, in locations within a mine where it is desired to remove a portion of a mine roof, bore holes are drilled through the portion of mine roof to be removed into the strata above. A conventional resin material is inserted in the bore hole ahead of a first ribbed bar, and the first ribbed bar is advanced upwardly into the bore hole. A second ribbed bar is secured to the first ribbed bar by means of a threaded coupling and advanced upwardly in the bore hole until the first ribbed bar is positioned in the bore hole above the portion of the mine roof to be removed. Both bars are then rotated within the bore hole to mix the resin material and chemically anchor the first ribbed bar in the bore hole. Before the portion of the mine roof is removed, the second ribbed bar is removed from the coupling, leaving the first ribbed bar chemically anchored in the bore hole above the portion of the mine roof to be removed.
U.S. Pat. No. 4,630,971 discloses an anchoring apparatus for insertion into a bore hole which includes a tension member in the form of an axially elongated steel rod with force transmission ribs on its outer surface and an anchoring element, such as a nut, engageable with the rod. The anchoring element has an inside surface with inwardly extending projections and the adjacent flanks on the projections form grooves within which the force transmission ribs engage. The anchoring element is formed of a higher strength material than the rod. When a predetermined axially extending tension force acting on the rod is exceeded, relative movement takes place between the anchoring element and the rod. The flanks on the projections on the anchoring element in contact with the ribs on the rod shear off a portion of the ribs contacted. The shearing action affords a sliding anchoring effect between the rod and the anchoring element.
Therefore, there is a need for an improved roof bolt system which utilizes an elongated ribbed bar or rod having helically extending rib segments roll-formed on the outer surface of the bar to provide a partial thread. The helically extending rib segments roll-formed on the outer surface of the bar provide a partial thread and eliminate the need for machine threading the end of the bar as with a conventional roof bolt. Furthermore, since the helically extending rib segments formed on the outer surface of the bar provide a partial thread over the entire length of the bar, ribbed bars may be provided which can be cut to length in the mine to suit particular application requirements. The ribbed bar must be capable of being utilized with a mechanical anchor including an expansion shell and an expansion plug having an internal bore defining a wall threaded to mate with the helically extending rib segments on the outer surface of the bar. Where combined mechanical and chemical anchoring of the ribbed bar in the bore hole is desired, the expansion plug includes a stop means positioned in the expansion plug bore to allow the ribbed bar and mechanical anchor to be rotated initially as a unit within a bore hole. The mechanical anchor and ribbed bar assembly must be capable of being inserted in a bore hole along with a suitable resin bonding material to provide a roof bolt system wherein combined resin bonding and mechanical anchoring of the ribbed bar in a rock formation is effected by continuous rotation of the ribbed bar in a single rotational direction to carry out both the operations of mixing the resin and expanding the shell. Where desired, the mechanical anchor and ribbed bar assembly must be capable of being inserted in the bore hole without resin bonding material to provide straight mechanical anchoring of the ribbed bar in the rock formation by continuous rotation of the ribbed bar in a single rotational direction to expand the shell into engagement with the bore hole wall. When straight mechanical anchoring of the ribbed bar in the bore hole is desired, a mechanical anchor is provided which does not utilize a stop means positioned within the expansion plug bore.